Ballooning instability preventing the H-mode access in plasmas with negative triangularity shape on the DIII-D tokamak

Infinite toroidal mode number (n = infinity) ballooning mode analysis of negative triangularity discharges on DIII-D shows that the access to 2(nd) stability becomes strongly restricted when the top triangularity decreases even modestly from -0.18 to -0.36. This is observed in experiment to coincide with the suppression of the L-H-transition. Further theoretical analysis with ballooning mode limited pedestals shows that the threshold for opening the 2(nd) stability access rises from a pedestal temperature of 0.3-1 keV when the top triangularity is decreased from -0.18 to -0.36 indicating that to access the 2(nd) stability with the more negative triangular shape would require unrealistically high pedestal temperature. The pedestal predicted by the EPED code agrees with the experimental H-mode profile for the negative triangularity case but in contrast to positive triangular shapes the prediction shows no increase in pedestal height with increasing core pressure when triangularity is negative. This work provides a first model to predict when negative triangularity plasmas can be expected to access the H-mode.

Automated summary

Analysis of negative triangularity discharges on DIII-D shows that decreasing the top triangularity restricts access to 2nd stability, coinciding with the suppression of L-H transition. Theoretical analysis indicates that accessing 2nd stability with more negative triangular shape would require unrealistically high pedestal temperature. Additionally, predictions suggest that increasing core pressure in negative triangularity cases does not result in an increase in pedestal height, contrasting with positive triangular shapes.